The treatment of fluids, both gases and liquids is well known and extensively practiced today. For example, filtration devices and their associated filtration elements are widely utilized in commercial, industrial and residential applications. The media or filtration element is very often located in an enclosed container which allows a contaminated fluid to be directed into the container, contact the filtration element, and then a now filtered fluid is directed out of the container. An important advantage of having the filtration element enclosed within a suitable container is that the container often entrains or otherwise captures the filtered contaminants and a spent element can be disposed as a unitary package providing for clean, fast, safe, and easy replacement. Liquid filter cartridges are used in many industries, for example as blood filters in the medical industry, as oil filters in engines, as fuel filters in fuel lines and tanks, and as water filters in refrigerators. Examples of gaseous filter cartridges include for example; air filters in furnaces, masks and canisters that are used to purify breathing air in respirators, automotive intake and catalytic exhaust filters, refrigerator air filtration, and whole room HEPA filtration. The design of the various filtration media and cartridges are well understood for each industry and application.
The treatment and/or filtration of fluids can be classified into several methodologies. Chemical treatments such as oxygenation, chlorination, and pH modification require the addition of chemicals to treat the fluid so as to change the nature of the contaminants allowing their inactivation or changing their morphology to enable subsequent mechanical filtration. Biological filtration uses microbes to convert the target contaminant into a form which either renders it safe, or binds it for subsequent mechanical filtration. Mechanical filtration which is the most common type, can be classified by the physical size of the contaminant to be removed. Traditional particle filtration can be used on particle sizes ranging from as high as 1000 microns down to 1 micron in size and includes contaminants such as, for example, sand, pollen, yeast, cysts, bacteria, pigments, and fine dust. A sand filter or spun polypropylene elements are representative of traditional particle filters. Filtering anything smaller that 1 micron typically involves porous hollow-fiber tubes or specialized membranes. Microfiltration ranges from 2 microns to 0.05 microns and can be utilized to target contaminants such as, for example, asbestos, smoke, and pigments. Ultrafiltration deals with the larger molecular level and ranges from 0.11 microns to 0.004 microns and can be utilized to remove contaminants such as, for example, carbon black, colloidal silica and viruses. Nanofiltration includes the smaller molecular level and includes particle size ranges from 0.009 microns to 0.0008 microns and can remove contaminants such as, for example, endotoxins, synthetic dyes, and sugars. Hyperfiltration has its domain in the ionic and atomic region from 0.0015 microns to 0.0001 microns (1 angstrom unit) and can include contaminants such as, for example, metal ions and salt. Reverse osmosis membranes can operate in the hyperfiltration range.
Mechanical filtration generally requires that the pore size of the media or element is smaller than the target contaminant. There are special challenges to using membranes having very small pore sizes as these membranes are easily fouled, have limited surface area and minimal capacity. Using multiple stages of pre-treatment or frequent backwashing extends the life of these membranes at the expense of complexity, cost, and wasted water. Moreover, many membrane systems operate at low flow rates and require very large membrane areas, high pressure supply pumps and/or storage reservoirs to provide usable amounts of water. An additional disadvantage to using membrane filtration is that because the filtered water is usually stored in reserve, it can support microorganisms which often foul the stored water and storage tank thus requiring additional filtration and or disinfection steps prior to use.
Another commonly used filtration technology is ion exchange which uses a positively charged resin (anion) or negatively charged resin (cation) to exchange one type of ionic contaminant for another. The most common example, a home water softener, uses a cationic resin which is initially saturated with sodium ions. When water containing dissolved ions such as magnesium and/or calcium is brought into contact with the sodium saturated cation resin, the more preferable calcium or magnesium cations are electrostatically bound to the resin and traded for sodium ions which are then released back into the water. Cationic resins target cations which are contaminants with a positive charge including transition metals such as calcium, magnesium, iron, aluminum, copper, mercury etc. Anionic resins target anions which are contaminants with a negative charge such as carbide, chloride, fluoride, oxide, sulfide etc. Even though ion exchange systems are well understood and quite common, they require backwashing and re-charging to maintain effective operation. The additional mechanisms and chemicals required to effectively employ ion exchange adds considerable cost and size for a suitable system. Additionally, while ion exchange resins can be used in replaceable cartridges, their cost is relatively high while possessing a generally low capacity for holding contaminants. As such, replaceable ion exchange cartridges have not found widespread economic success in the marketplace.
One particular replaceable filtration cartridge niche that has shown remarkable economic success and widespread use is in the water filtration systems of many refrigerators. In 1996, KX industries, was the first to introduce a water filter for use with a refrigerator. By 1998, most domestic refrigerator manufactures were offering integrated replaceable water filter systems. U.S. Pat. No. 6,193,884 to Magnusson et al. teaches the use of a replaceable water filter suitable for use in an appliance. These early water filters typically employed carbon granules and were capable removing chlorine, some organics, and were able to reduce turbidity from water to improve its taste and clarity. Advances in carbon block technology today utilize a polymeric sintered matrix of ultrahigh molecular weight polyethylene with very fine powdered activated carbon and specialized additives such as amorphous titanium silicate (ATS). U.S. Pat. No. 7,112,272 to Hughes et al. discloses using two special PE polymers and vibration sintering to produce a very efficient, structurally robust, and high performing filtration element. U.S. Pat. No. 7,293,661 to Saaski et al., discloses a 2-part binder agglomerated particle using UHMW-PE and activated carbon. These types of carbon and polyethylene filtration elements are now capable of removing 98% of volatile organic compounds, 97% of chlorine, 99% of lead, 96% of mercury, 99% of asbestos, 99.99% of cysts, 99% of lindane, and 74% of atrazine while flowing at 0.6 gpm for 160 gallons as listed by NSF, which is very respectable considering their very small size. However, federal, state, and local governments are continuing to impose stricter regulations on maximum contamination limits to an ever growing list of toxic contaminants.
Many specialized water treatment systems are capable of targeting difficult contaminants such as perfluorochemicals (PFC), particularly perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), chlorine byproducts such as trihalomethanes (THM), nitrogen-oxygen compounds nitrate and nitrite, and heavy metals such as antimony, arsenic, bismuth, cadmium, cerium, chromium, cobalt, copper, gallium, gold, iron, lead, manganese, mercury, nickel, platinum, silver, tellurium, thallium, tin, uranium, vanadium, and zinc, and even naturally occurring radionuclide's such as uranium, plutonium, radon, and radioactive fallout such as thorium, barium, cerium, caesium, tellurium, ruthenium, molybdenum, strontium, lanthanum, and iodine. However, it is has been very difficult to provide a simple, low-cost filter cartridge to reduce many of these contaminants in meaningful amounts. A recent study according to the Associated Press has disclosed that over 56 different prescription drugs like psychoactive anti-anxiety medications, pain medications, sex hormones, and antibiotics etc. have been found in over 24 major metropolitan area water supplies. It is very difficult and expensive to reduce these and other contaminates to acceptable levels in an economical replaceable filter cartridge.
Electrical interactions play a large role in chemical activity and can be utilized to assist or promote fluid treatment. The oxidation or reduction of a chemical or molecule is a transfer of electrons from one atom to another. It can be said that the oxidation of iron (iron looses electrons) is also the reduction of oxygen (oxygen gains electrons). This process is often called redox and can be measured in terms of a redox potential or voltage. As iron is dissolved (reduced) into water it forms what is called clear-water or ferrous iron as it gains electrons.(Iron Reduction) Fe→Fe2++2e−
As iron is oxidized, it precipitates into a larger red-oxide colored molecule as it loses electrons.(Iron Oxidation) 4Fe2++O2→4Fe3++2O2−
These interactions and reactions involve atoms, molecules, and chemicals that are considered ions because they have either a net loss of electrons, or a net surplus of electrons. Ions are by definition polar and have a distinct electrical charge. An ion that has extra electrons is negatively charged, while an ion that is deficient in electrons has a net positive charge. It is worth noting that almost all pathogenic microbes are positively charged. For example, when sodium chloride salt is dissolved into water, it becomes sodium cation and chlorine anions.
Chlorine bleach or sodium hypochlorite is often used to treat organics which contaminate water. It is well known that a certain concentration of available chlorine (ppm) will destroy a certain amount of bacteria within a know amount of time. What is less understood is that the available chlorine in ppm is also equal to a redox potential or voltage of approximately 0.69 volts. In other words, the electrical potential of the liquid is changed by the addition of the bleach and has a new ability to steal electrons from substances dissolved in the water such as the bacteria. The use of a redox meter can be used to determine the ability of a liquid to oxidize a substance. Further, it matters little to the substance being treated if the redox potential is the result of bleach, acid, hydrogen peroxide, ozone and fluorine or an induced voltage. The ability of the liquid to steal electrons is based on the voltage between the liquid and the voltage of a newly introduced species.
A Chart of the Relative Values in Volts Oxidation-Reduction
ChemicalSymbolOPR Relative ValueFluorineF2.25Hydroxyl RadicalOH2.05Oxygen (Atomic)O11.78OzoneO31.52Hydrogen PeroxideH2O21.30PermanganateKMn21.22Hypochlorous AcidH2CL1.10Chlorine (Gas)CL1.00Oxygen (molecular)O20.94Sodium HypochloriteNaCL20.69BromineBr0.57
Ion exchange resins filter fluids with these same electrical reactions because the plastic resin which is normally a cross-linked polystyrene bead, has been doped with a charged molecule (functional group) such as quaternary ammonium or divinyl-benzine to give the resin beads a lasting charge or voltage without external bias.
Electrolysis is also another process by which contaminants can be both chemically and physically altered to facilitate mechanical filtration. During electrolysis, a bias voltage is placed across two electrodes immersed in the treatment fluid which is called an electrolyte. The fluid must have some number of ionic contaminants such that the fluid can become electrically conductive. Dissolved salts allow electrons to be conducted between the cathode and anode. When these electrodes are connected to each other, galvanic corrosion can occur which is common in boats with marine drives in sea water. When the electrodes are connected to an external bias voltage such as from a battery, the galvanic process can be augmented or reversed based upon the polarity of the electrodes and electrode materials. Galvanic interaction can cause metals to come out of solution or plate onto electrodes. The electrodes can be of any suitable conductive material such as carbon, graphite and metals.
Electrodialysis (ED) is a specialized and complex system in which membranes are used to separate fluid streams of anodic and cathodic fluids to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. It can be likened to reverse osmosis with electrolysis. Another method employing membranes is electrodeionization (EDI) where membranes and ion exchange resins are used to separate the water into anodic and cathodic concentrates which form a waste stream to be discarded or recirculated. This system is likened to mixed-bed DI with electrolysis. These systems are effective, but are also slow in operation and costly to purchase and maintain and go beyond the economics of this application being best suited for laboratory, microelectronics, pharmaceutical, and industrial processing of ultra-pure fluids.
When water is subjected to electrolysis, the electron transfer process disassociates the H2O molecule into namely oxygen and hydrogen gasses. These gases dissolve into the water and can even super saturate the water as disclosed in U.S. Pat. No. 6,689,262 to Senkiw. Oxygenated water can be used to precipitate metals such as iron, manganese and even arsenic. Further, the electrolysis of water produces a wide range of transient high-energy chemical hybrids and interactions which can instantly precipitate dissolved metal. These electrolytic reactions produce atomic oxygen (O1) and atomic hydrogen (H1) which eventually lower their energy state to O2 and H2 as they pair and become stable. Additionally, ozone, hydrogen peroxide, hydrogen and hydroxyl radicals are understood to exist in transient stages which interact aggressively with contaminants in the water. For example, metal ions such as ferrous iron can enlarge 4-5 orders of magnitude from 0.0005 micron size of the dissolved ionic metal into a precipitated particle size of 1 to over 50 microns. Electrolysis can be used to liberate additional gasses such as chlorine from ionic chlorides dissolved into water also and can be beneficial to purify microbiological contaminants in the manner that swimming pool chlorinators work.
There is another electrical phenomenon which is involved with colloidal suspensions. A voltage exists between suspended particles in a colloid and their dispersion medium (water). This electrical voltage is called the zeta potential and measures the repulsion between particles such that they will remain separated and not coalesce into a larger agglomeration. When colloids are in the presence of a greater static electrical field, the zeta potential can be eliminated or altered such that the particles will coalesce into larger particles and become self clarifying by settling or becoming large enough to be removed by particle filtration. U.S. Pat. No. 4,007,113 to Ostreicher teaches the making of a filtering device by electrically modifying the zeta potentials using melamine-formaldehyde. Unfortunately, melamine-formaldehyde is not safely used for potable drinking water media.
There have been many different devices made that use one or more of these electrical processes to reduce contaminants in fluids, both gaseous and fluidic, but fail to accomplish the stated goal of providing a workable economic unitary package that allows for its clean, fast, safe, and easy replacement. U.S. Pat. No. 3,616,356 to Roy discloses an invention for the electrolytic treatment of water containing dissolved salts and metal oxides. The device placed planar electrodes opposite a bed of particulate carbon wherein a voltage of at least 1 volt was able to pass current of several amps between the carbon anode and steel cathode. The device was able to reduce dissolved metals by way of electrolysis and reclamation was by subsequent filtration of the fluid. This batch type device fails to provide continuous service and because of high current electrolysis, significant gasses will be generated and require their venting. Further, Roy teaches that using fine carbon powders defeats the purpose of the device because the metal pates to the carbon and cannot be easily removed.
In U.S. Pat. No. 4,941,962, Inoue discloses an invention for the electrostatic adsorptive treatment of fluids where he teaches a charged metallic housing electrode arranged opposite to an internal center electrode sleeve separated by an adsorbent filter media (carbon, zeolite, clay, activated alumina). A voltage is impressed across the outer housing and inner sleeve electrodes, from 1-20 VDC per cm. Inoue's invention claims that it manipulates and augments Coulomb forces which affect the charge between the impurity to be filtered and the surface of the adsorbent. Additionally, the device erases the zeta potentials of impurities as they pass by the charging electrode causing the particles to cohere together and settle. Those particles remaining are attracted by intensive Coulomb forces and are attracted to the holes of the adsorbent where they are retained until polarity reversal. Thus the Inoue device greatly increases both the strength and the capacity of the media adsorption. This device is an improvement over Ostreicher in that it is a flow-through housing, but falls short by not being an easily replaceable, low cost cartridge. However, it is Inoue's assertion that the use of an adsorbent which becomes saturated cannot be reused and must be thrown away, therefore requiring the reversal of polarity to release the contaminants and discharge them. The fact that contaminants remain on the saturated adsorbent are ideal for a replaceable filter cartridge which is in contrast to Inoue's device.
In U.S. Pat. No. 5,164,091, Huber et al., teaches how to remove metal ions in waste water by using an electrically conducting, cathodically polarized filter-aid layer whose potential is at least 50 mV more negative than the redox potential of the metal ions to be removed. The filter-aid layer is comprised of metal granules and carbon. Unfortunately, Huber's device requires ionic selective membranes and frequent backwashing for functionality. It is of particular interest however to define a minimum negative voltage to effectively interact with any particular metal ion.
Electrochemical Series for Some Metallic Ions as Referenced to Hydrogen
Metallic IonE° (volts)Lithium−3.03Potassium−2.92Calcium−2.87Sodium−2.71Manganese−2.37Aluminum−1.66Zinc−0.76Iron−0.44Lead−0.13Hydrogen (2H+)0.00Copper0.34Silver0.80Gold1.50From the chart, a value of −3.03+50 mV [3.053 volts] would be sufficient to interact with lithium.
In U.S. Pat. No. 5,281,330, Oikawa et al., discloses a battery operated device with an electrically conductive filter within a water channel to suppress the breeding of microorganisms. The conductive filter is connected to a circuit board to carefully control the limited amount of battery power such that the device can last as long as possible and that one of the electrodes must be electrically insulated to eliminate any current flow. Particularly, the voltage is disabled during water flow because microorganisms do not typically breed as they are being flushed. Oikawa determined that a minimum of 0.7 volts was sufficient to stop the breeding of bacteria.
In U.S. Pat. No. 6,332,960, Monteith teaches a device to purify fluids, both liquid and gases by using a flow-through housing which has bolt-on flanges. While the device is removable, Montieth does not teach the need for easy low-cost replacement nor the use of adsorbents. Further, Montieth's device requires voltages in the kilovolt range, well beyond any safety for a residential filter cartridge use. However, Monteith's invention uses static electricity to charge inorganic particles such that they coalesce into spherical aggregates. Additionally, the electrostatic charge is effective in killing biological contaminants.
In U.S. Pat. No. 6,673,321, Weakly teaches the use of an apparatus that uses very high voltages (5000 volts) with only a trickle of current (0.5 to 3 mA) to enhance adsorption, polarization, ion exchange, and to agglomerate dissolved metal. Weakly impresses upon us that because the electrodes are insulated and the resulting current is so low, the process does not use electrolysis, but instead metals are captured by subsequent filtration by an adsorbent.
In U.S. Pat. No. 7,622,025, Polnicki discloses a system for decontamination of fluids using graphite, aluminum, or iron electrodes. Polnicki demonstrates that heavy metals such as arsenic, hydrocarbons, tensides (detergent agents), phosphates, chlorine aromatics and even bacteria are able to be removed by electrolytic activity. While iron is effective at conducting electricity, iron is very sacrificial and will not survive long due to galvanic and corrosive attack. Aluminum is also not recommended for potable water according to NSF. Graphite, as defined in this disclosure is 99% pure carbon but it is not activated such that it is intended for a filtration media. Activated carbon is highly porous and as such, has very large surface area.
In U.S. Patent Publication No. 2011/0042236 A1, Jae-eun Kim et. al. advances a drinking water filter unit that is able to sterilize microorganisms by applying an alternating polarity voltage to a filter layer using electrodes where the filter layer can be rolled into a spiral geometry. While Jae-eun Kim et al. teaches that a filter layer in a spiral geometry is an effective mechanism to enhance surface area and provide multiple polar regions, his teaching requiring that in all cases, the voltage be alternately reversed. Each time the poles are reversed, materials and contaminants entrapped onto the electrically enhanced media, are repelled and can be released causing bleed-through and downstream contamination.
While there exist many variations on fluid treatment systems, what is still needed is a low cost means to augment the capacity of filtration media, expand the number of contaminants that are possible to reduce, and increase the filtration kinetics while at the same time keeping the advantages of clean, fast, safe, and easy cartridge replacement.